Recently, wireless charging systems have been largely implemented in two methods. One method is a tightly coupled method in which an antenna of a power transmitting unit (PTU) and an antenna of a power receiving unit (PRU) should be positionally matched and close to each other. This method has good efficiency because an operating frequency is low and the antenna of the PTU and the antenna of the PRU are positioned within a short distance of each other and are comparatively accurately matched to each other, and also has an established control method because a control method thereof is similar to a conventional resonant inverter method. As a standard adopted by such a technology, the Power Matters Alliance (PMA) method and the Qi method of the Wireless Power Consortium (WPC) exist.
The Qi method and the PMA method have good efficiency and comparatively low production costs. However, it is difficult for a single PTU to simultaneously supply energy to two or more PRUs, and also charging efficiency is rapidly reduced when an antenna of a PTU and an antenna of a PRU are not positionally matched. Also, even when a distance between the PTU and the PRU increases slightly, charging efficiency decreases, and thus the above-described methods are inconvenient for general users to use.
In order to solve these problems, unlike the above-described standard, a standard capable of performing a charging action regardless of distance and position by increasing an energy transmission frequency by using magnetic resonance technology has been proposed. The Alliance for Wireless Power (A4WP) is a representative standard thereof. While the Qi and PMA methods transmit energy at a frequency ranging from about 80 kHz to about 200 kHz, the A4WP may have a small antenna at a high frequency of 6.78 MHz and may be capable of wireless charging even when antennas are separated from each other by matching resonant frequencies of resonators of a PRU and a PTU. By using the A4WP method, it is not difficult to supply several Watts of power in an area of several cm2, and it is also possible to simultaneously transmit energy to several PRUs. This method is called a loosely coupled method. However, this method has a high operating frequency so that implementation is difficult and also has lower efficiency than a tightly coupled method because active elements are driven at a high frequency.
The Qi, PMA, and A4WP methods each have advantages and disadvantages, and thus the three methods are mixed and used. However, the standards have different frequencies, and thus wireless charging standards thereof are not compatible with each other. Accordingly, there is a need for a PRU capable of receiving wireless power from PTUs of any standard.